1. Field of the Invention
The present invention relates to a dielectric element including a dielectric layer and electrodes, such as a piezoelectric transformer, a SAW (Surface Acoustic Wave) element, an electron emitter or the like, and production methods of the dielectric layer and the dielectric element.
2. Description of the Related Art
With regard to such dielectric elements, various materials have been studied as materials composing a dielectric layer with the aim of the improvement of the performance, the downsizing of the element, and the reduction of the cost.
For example, a piezoelectric transformer is widely known as such a dielectric element. The piezoelectric transformer includes a dielectric layer, an input electrode, an output electrode, and a ground electrode. The input electrode and the ground electrode are formed in parallel with the surface of the dielectric layer and disposed so that the electrodes face each other at a prescribed interval. A dielectric material that composes the dielectric layer intervenes between the input electrode and the ground electrode, and the dielectric layer is prepared so as to mechanically vibrate (so that a stress is generated in the dielectric layer) by inverse piezoelectric effect caused by applying pulse-shaped input voltage between the input electrode and the ground electrode. The output electrode is formed for example at the end face of the dielectric layer and prepared so as to output the voltage generated between the output electrode and the ground electrode by piezoelectric effect caused by the mechanical vibration or the stress.
Such piezoelectric transformers can be used in various kinds of applications such as: a hot-cathode tube inverter used for a fluorescent lamp or the like; a backlight cold-cathode tube inverter for a liquid crystal display used for a large-screen liquid crystal television, a notebook personal computer, a portable remote terminal or the like; an AC adapter and a DC-DC converter used for various kinds of electronic equipment including various kinds of high-voltage transformer assembly, a negative ion generator, an air cleaner and the like; and others.
As a dielectric layer composing the piezoelectric transformer, PZT ceramics (refer to JP-A No. 116205/1997) and lithium niobate monocrystal (refer to JP-A No. 191160/1996) have heretofore been generally known. The piezoelectric transformer, which uses the PZT ceramics, disclosed in JP-A No. 116205/1997 contains Pb which is a material hazardous to environment and hence the control of Pb has been necessary in the production processes. In this light, a piezoelectric transformer having a lead-free dielectric layer as described in JP-A No. 191160/1996 has actively been studied in recent years.
However, in the case of the piezoelectric transformer, which uses the PZT ceramics, described in JP-A No. 116205/1997, the mechanical quality factor Qm of the PZT ceramics is small (3,000 to 4,000). Therefore, the change of properties and the loss of electric power caused by the heat generation of the dielectric layer increase and hence the piezoelectric transformer has not been used for a high electric power application of 5 W or more and it has also been difficult to increase the step up ratio.
Meanwhile, there have been various problems also in the case of a piezoelectric transformer, which uses lithium niobate monocrystal in order to reduce the environmental load and realize a high step up ratio and high efficiency by a large Qm (10,000 or more), disclosed in JP-A No. 191160/1996.
For example, the lithium niobate monocrystal is more expensive than polycrystalline materials such as PZT ceramics and the like. Further, if it is attempted to form a thinner dielectric layer in order to downsize an element or increase the step up ratio by forming a multilayer, microcracks have been generated, thus the yield has deteriorated, and the cost has further increased when wafers are cut out from a monocrystal ingot.
Furthermore, when the piezoelectric transformer is used for high electric power applications, there has been the possibility that the stress generated in the dielectric layer due to mechanical vibration concentrates at the nodes of the mechanical vibration and the dielectric layer breaks at the nodes of the vibration. Therefore, it has been necessary to increase the thickness of the nodes of the vibration where the stress concentrates, and expand the width of the substrate in order to make the cracks of the dielectric layer hardly generated even in the high electric power applications. Consequently, the downsizing and profile reduction of the piezoelectric transformer have been difficult.
In addition, when the thickness of the dielectric layer is increased in order to make the aforementioned cracks hard to be generated, the operating impedance increases. As a consequence, in this case, it has been necessary to boost the piezoelectric transformer by introducing a wound transformer or the like to the front of the piezoelectric transformer and hence the downsizing and profile reduction of the piezoelectric transformer have further been difficult. Further, when the width of the substrate is expanded, spurious vibration has occurred and the efficiency has lowered.